Cutting of glass is conventionally accomplished by using mechanical tools; however, an alternative process exists that uses CO2 laser radiation at a wavelength of 10.6 μm to heat the glass and create tensile stress via a temperature gradient. The use of a CO2 laser for glass cutting is discussed in: Kondratenko, U.S. Pat. No. 5,609,284 entitled “Method of splitting non-metallic materials” (the '284 patent); commonly-assigned Allaire et al., U.S. Pat. No. 5,776,220 entitled “Method and apparatus for breaking brittle materials” (the '220 patent); Ostendarp et. al. U.S. Pat. No. 5,984,159 entitled “Method and apparatus for cutting through a flat workpiece made of brittle material, especially glass” (the '159 patent); and commonly-assigned Allaire et al., U.S. Pat. No. 6,327,875 entitled “Control of median crack depth in laser scoring” (the '875 patent). The contents of the commonly-assigned '220 and '875 patents are incorporated herein by reference in their entireties.
As shown in FIG. 9, during laser scoring a median crack 115 (also known as a partial vent or, simply, a vent) is created in a major surface 114 of a glass sheet 112. In order to create the vent, a small initial flaw 111 is formed on the glass surface near one of its edges, which is then transformed into the vent by propagating laser light 121 formed into a beam 113 across the surface of the glass followed by a cooling area produced by a cooling nozzle 119. Heating of the glass with a laser beam and quenching it immediately thereafter with a coolant creates a thermal gradient and a corresponding stress field, which is responsible for the propagation of the vent.
Laser beams having various shapes and sizes have been employed in the patents referred to above. The scoring beam described in the '284 patent had a short elliptical shape with the longest axis of the beam being less than 10 times the material thickness. In accordance with this approach, in the case of a glass sheet having a thickness of 0.7 mm, the typical thickness of a display substrate, the length of the beam's long axis should not exceed 7 mm. In the '220 patent, the scoring beam had an elongated elliptical shape with the longest axis preferably greater than 40 mm. In the '875 patent, the beam was truncated from one or both ends, and as a result, the total length of the beam was reduced by 20-40%. In the '159 patent, a U-shape beam created by a scanning technique was used for scoring.
A variety of scoring speeds are described in the above patents ranging from a low of 6 mm/s in the examples of the '284 patent to 1000 mm/s for the complex beam structure of the '159 patent. Significantly, none of these references contain any mention of the problem of residual stress produced in glass sheets as a result of laser scoring. As such, the references are completely silent as to the problem of increased residual stress which, in accordance with the invention, has been found to accompany increases in scoring speed.
Residual stress is a particularly significant problem in the case of glass sheets that are to be used as substrates in display devices. Many display devices, such as TFT-LCD panels and organic light-emitting diode (OLED) panels, are made directly on glass substrates. To increase production rates and reduce costs, a typical panel manufacturing process simultaneously produces multiple panels on a single substrate or a sub-piece of a substrate. At various points in such processes, the substrate is mechanically divided into parts along cut lines.
Such mechanical cutting changes the stress distribution within the glass, specifically, the in-plane stress distribution seen when the glass is vacuumed flat. Even more particularly, the cutting relieves residual stresses in the sheet at the cut line since the cut edge is rendered traction free. Such stress relief in general results in changes in the vacuumed-flat shape of the glass sub-pieces, a phenomenon referred to by display manufacturers as “distortion.”
Although the amount of shape change as a result of stress relief is typically quite small, in view of the pixel structures used in modern displays, the distortion resulting from mechanically cutting individual panels out of a larger sheet can be large enough to lead to substantial numbers of defective (rejected) displays. Accordingly, the distortion problem is of substantial concern to display manufacturers and specifications regarding allowable distortion can be as low as 2 microns or less.
The amount of distortion produced when such mechanical cutting is performed depends on the residual stress in the sheet, with lower levels of residual stress producing smaller distortions. As discussed above, the prior art relating to laser scoring is silent with regard to residual stress introduced into glass sheets during the scoring process. As such, the prior art is also silent with regard to the distortion resulting from such residual stress when the glass sheets are subsequently mechanically cut during panel manufacturing processes.
In addition to the distortion problem, as discussed below, residual stress is also important in terms of the quality of the edges produced when a laser scored sheet of glass is divided into two sub-pieces. In accordance with the invention, high levels of residual stress have been associated with edges having relatively low strength and poor quality, e.g., splinters and micro cracks. It has also been found that high residual stress nearby the glass edge may cause a gradual deterioration of the edge quality, namely chipping or delamination, which manifests itself some time after scoring or can be induced by an external impact. Again, the prior art is silent with regard to these problems with laser scoring.
A third problem with the prior art techniques for performing laser scoring relates to the CTE of the glass being scored. Prior art laser scoring techniques have used glasses with relatively high CTE's, e.g., Corning Incorporated's Code 1737 LCD glass which has a CTE (0-300° C.) above 37×10−7/° C. More recent glasses, e.g., Corning's EAGLE2000® and EAGLE XG™ glasses, have lower CTE's. Higher CTE's, such as that of Code 1737 glass, translate into higher tensile stresses during heating which, all other things being equal, means that it easier to laser score such glasses at higher speeds. The lower CTE's of the more modern glass substrates used by the LCD industry result in much lower scoring speeds when conventional laser scoring technology is used.
In view of these various problems, there exists a need in the art for processes that can provide high speed laser scoring of glasses having lower CTE's (i.e., CTE's less than 37×10−7/° C. (0-300° C.)) and at the same time do not generate excessive residual stresses.